I trim most all my pond plants in the spring (except lillys) for several reasons.
1. When the plants die back, the leaf material will settle at the water level and make a natural layer of winter protection. When the snow falls on top of the hump of dead plant material it will add further winter protection and provide better "air flow" to the water in the deep winter, as the leaf litter and dead material on the surface of water are naturally pourus and let more air flow to under the ice. I read in a book that people used to put a bale of hay in the dugout years ago to provide air and oxygen to the fish under the water. I also noticed that my fish winter each year under the roots of my very large floating yellow flag, I think it must be a good idea, if the fish all winter there, and it makes sense to me. The dead material lays on top of the water, and I crumple it up over the plant, and let the snow fall over it, kind of like a winter blanket.
2. The dead stuff comes off the plant much easier in the spring!
3. It's natural.... ever notice the cat tails and rushs that grow naturally along the creek come back so quick and look so lush, and no body every cleans the dead tops of ever!! The plants come back so quick because the energy went back to the bulb in the fall naturally, and the flowing water all year round under the ice helps too. The dead stuff will protect the plant from spring frost even well into the spring, as the new shoots pop up from under the ice and snow.